Sintered metal article and method of improving its fatigue resistance by shot reening



Nov. 6, 1956 c. R. TALMAGE 2,769,709

SINTERED METAL ARTICLE AND METHOD OF IMPROVING ITS FATIGUE RESISTANCE BY SHOT PEENING Filed Jan. 7, 1953 INVENTOR Click/es Reeri 75/010942 BY 5y. v ELI/"Z0404 ATTORNEYS all" United States Patent SINTERED METAL ARTICLE AND METHOD OF llVIPROVlNG ITS FATIGUE RESISTANCE BY SHOT PEENING Charles Robert Talmage, New Canaan, Conn.

Application January 7, 1953, SerialNo. 329,983

2' Claims. (Cl. 75-221) Today, many parts are made from metal powder, such, for example, as iron powder in which: the powder is pressed to shape and then sintered. In; some instances the part is given a second. pressing; after the sinteri-ng. Such parts are used for many purposes-but one. criticism of them is that they do not always have adequate strength and do not have a suificiently long life under conditions of fatigue. They may tend to crack after continued use under conditions where they are subjected to strains. of the type which tend to exert surface stretching, as for example under conditions tending to. flex the member;

This invention relates to a method whereby such parts made from powder metal can be given improved strength and fatigue life. Briefly, my invention comprises the steps of submitting the surface where fatigue cracks will tend to start, to a peening operation, and specifically to shot peening, after which the part is again sintered at a high temperature. For example in the case of iron, I may use a temperature in the order of about 2050 F. Other temperatures as commonly used for sintering powder metal forms may be used.

Shot peening is an operation in which the surface of the metal part is mechanically worked by subjecting it to blows from fine balls or other shot-like bodies which are forced against the surface at high velocity as by air pressure. This operation is well known in the art and apparatus for shot peening can be purchased in the open market.

It is recognized that for ordinary metal parts such as those formed by stamping or the like from solid metal, shot peening in and of itself gives some increase in fatigue life but this is commonly attributed to the fact that the impact of the balls tends to compress the surface of the part so that such surface remains under strain. Subsequently when the surface is subjected to an operation tending to stretch it, these compressive strains tend to give the surface of the body an increased modulus of elasticity. Because this is the effect desired, care is taken to have the shot peening a final operation and the part is used without giving it any exposure to heat which would tend to release the strains set up by the peening operation.

When used in accordance with my invention, the shot peening has an entirely different function. Parts formed from powdered metal, even though formed under great pressure, when examined under a microscope will be found to have minute pores throughout their mass. Those within the body of the part do relatively little harm but some of these pores extend out to the surface and in such case they tend to form points of weakness where fatigue cracks will start. I have found that when such a surface is subjected to shot peening, the mechanical treatment tends to close the metal around the pore openings and thus give the part a comparatively solid surface relatively free of pores.

Under normal conditions the peening operation does not itself cause the particles of metal to weld together "ice strongly across these pore openings, but when the part is subjected to a subsequent sintering, the peened particles do increase. their bond strength thus tending to form a part. with a virtually continuous unified surface largely free from open pores where fatigue cracks may start. The sintering operation serves fully to anneal the member so there are none of the inherent strains left as a result of the peening.

The effect described is illustrated in the accompanying microphotographs in which Fig. 1 is a view enlarged 100 times of an edge of a bushing to be used in a roller chain which is made from powdered metal and. finished in the usual manner.

Fig. 2 is a similar view of a similar bushing which has been subjected to my process.

Referring to Fig. 1, the. white portion of the figure is the bushing itself and the black is backing material in which it is imbedded for the purpose of increasing visibility. It will be noted that the white material has open pores in the formof substantial gaps. or crevices in its. surface which show in the. illustration along the edge of the white body. Under fatigue conditions, any of these could serve as spots where. cracks could start.

In Fig. 2 which corresponds: to Fig; 1 (though not.

of the identical section) it will be noted that. there are no pore openings extending to; the surface. Even if some cracks are present they are reduced in size- It should be noted that near each side of Fig. 2 the material has a pore removed a short distance back from the surface or edge. These were probably open pores before the part had been given the shot peening and sintering treatment. This is particularly noticeable near the extreme lefthand side of this figure where the closing of the aperture is quite evident.

The manner in which my invention is to be carried out is illustrated by the following example:

In this case I took powder metal bushings compacted from a sponge iron powder consisting of approximately 94 parts iron, 5 parts copper, and 1 part graphite powder. The part had been compacted in an automatic press under a pressure of approximately 4 tons and was sintered after compacting in accordance with conventional practice.

Using my process, these powder metal bushings were placed in an air blast type shot peening machine and subjected to a peening operation using -inch diameter steel balls. The balls were forced through the nozzle of the shot peening equipment at an air pressure of approximately pounds per square inch and the bushings were held approximately 3 inches from the nozzle and were continuously rotated during the shot peening operation. The bushings were subjected to shot peening for a period of approximately 5 minutes and then removed from the chamber, placed on a tray and inserted in a mufile furnace. The temperature of the furnace was held at 2050 F. and the bushings were held at this temperature for 15 minutes. The bushings were kept in the muffle furnace for a total period of 1 /2 hours and were held at a temperature of at least 1500 F. for at least of an hour.

Tests were run to ascertain the porosity of the surface of bushings which had been treated by my process and like bushings which had not been so treated. These tests consisted of the following steps:

Certain of the bushings without shot peening but formed and sintered in the usual manner of forming metal powder parts were hardened by conventional heat treating methods and were impregnated with oil and then tested for fatigue life. Other similar bushings after sintering and hardening were shot peened, and then without re-sintering were impregnated with oil and tested. A third group of these bushings were sintered, shot peened,

2,769,709 y 4 H p 3 re-sintered and then hardened, impregnated and tested. The first set of bushings under test conditions of strain showed a fatigue life of about 10 hours; the second set under similar test conditions showed a fatigue life of about 30 hours, and the material which was shot peened and then sintered as provided for in this application showed a fatigue life of about 75 hours or more.

The amount of peening to be employed is ordinarily judged by an examination of the surface of the part and should show that the entire surface has been worked over by the action of the balls which give it a somewhat pebbly appearance. The second sintering is carried out in a manner comparable to the sintering ordinarily employed after the compression operation of a part made from metal powder and similar times and temperatures are normally used.

While my invention is particularly useful in connection with parts made from iron powder, it may also be employed with mixed metals for forming alloys or with metals other than iron such as bronze, copper or the like. Ordinarily my process will be used with powder metal parts that have been compressed and sintered but it can also be used to increase the surface density of a mass of sintered powder even where little or no compression has been employed. This application of my process appears to be particularly useful in connection with masses made from a type of powder that sinters readily, as for example masses made from powder containing copper.

What I claim is: r

1. In the process of forming a part of substantially finished shape from powdered metal comprising metal selected from the group consisting of iron, copper and bronze by compressing themetal powder to desired form and then heating it to cause the particles to sinter together, the steps of shot peening the surface of such part and thereby pushing together edges. of pores on the surface of such part and thereafter without substantially changing the shape of the part again heating such part to the V sintering temperature and sintering together pore edges brought together by the shot peening step whereby a part is produced substantially ready for use in its finished desired form and characterized by its high resistance to fatigue.

2. A metal part consisting of compressed and sintered metal powder comprising metal selected from the group consisting of powdered iron, copper and bronze having on its periphery a layer of compacted metal showing on its outer surface the pebbly appearance resulting from 7 shot peening and having surface pore areas in which the edges of such areas are hammered together by the shot peening operation and are sintered together with heat, such outer surface being substantially free from internal stresses set up by shot peening and said part being characterized by its strength and resistance to fatigue.

References Cited in the file of this patent UNITED STATES PATENTS 2,478,856 Beaver Aug. 9, 1949 

1. IN THE PROCESS OF FORMING A PART OF SUBSTANTIALLY FINISHED SHAPE FROM POWDERED METAL COMPRISING METAL SELECTED FROM THE GROUP CONSISTING OF IRON, COPPER AND BRONZE AND COMPRESSING THE METAL POWDER TO DESIRED FORM AND THEN HEATING IT TO CAUSE THE PARTICLES TO SINTER TOGETHER, THE STEPS OF SHOT PEENING THE SURFACE OF SUCH PART AND THEREBY PUSHING TOGETHER EDGES OF PORES ON THE SURFACE OF SUCH PART AND THEREAFTER WITHOUT SUBSTANTIALLY CHANGING THE SHAPE OF THE PART AGAIN HEATING SUCH PART TO THE SINTERING TEMPERATURE AND SINTERING TOGETHER PORE EDGES BROUGHT TOGETHER BY THE SHOT PEENING STEP WHEREBY A PART IS PRODUCED SUBSTANTIALLY READY FOR USE IN ITS FINISHED DESIRED FORM AND CHARACTERIZED BY ITS HIGH RESISTANCE TO FATIGUE. 